Everything about Photolysis totally explained
Photodissociation,
photolysis, or
photodecomposition is a
chemical reaction in which a
chemical compound is broken down by
photons. Photodissociation isn't limited to
visible light, but to have enough
energy to break up a molecule, the photon is likely to be an
electromagnetic wave with the energy of visible light or higher, such as
ultraviolet light,
x-rays and
gamma rays. The direct process is defined as the interaction of one or more photons interacting with one target molecule.
Photolysis in photosynthesis
Photolysis is a part of the
light-dependent reactions of
photosynthesis. The general reaction of photosynthetic photolysis can be given as:
H
2A + 2 photons (light)
2e
- + 2H
+ + A
The chemical nature of "A" depends on the type of organism. For example in
purple sulfur bacteria,
hydrogen sulfide (H
2S) is oxidized to sulfur (S). In oxygenic photosynthesis, water (H
2O) serves as a substrate for photolysis resulting in the generation of free oxygen (O
2). This process is responsible for generating the majority of breathable oxygen in earth's atmosphere. Photolysis of water occurs in the
thylakoids of
cyanobacteria and the
chloroplasts of
green algae and
plants.
Energy transfer models
The conventional,
semi-classical, model describes the photosynthetic energy transfer process as one in which excitation energy hops from light-capturing pigment molecules to reaction center molecules step-by-step down the molecular energy ladder.
The effectiveness of photons of different wavelengths depends on the absorption spectra of the
photosynthetic pigments in the organism.
Chlorophylls absorb light in the violet-blue and red parts of the spectrum, while
accessory pigments capture other wavelengths as well. The
phycobilins of red algae absorb blue-green light which penetrates deeper into water than red light, enabling them to photosynthesize in deep waters. Each absorbed photon causes the formation of an
exciton (an electron excited to a higher energy state) in the pigment molecule. The energy of the exciton is transferred to a
chlorophyll molecule (
P680, where P stands for pigment and 680 for its absorption maximum at 680 nm) in the reaction center of
photosystem II via
resonance energy transfer. P680 can also directly absorb a photon at a suitable wavelength.
Photolysis during photosynthesis occurs in a series of light-driven oxidation events. The energized electron (exciton) of P680 is captured by a primary electron acceptor of the photosynthetic
electron transfer chain and thus exits photosystem II. In order to repeat the reaction, the electron in the reaction center needs to be replenished. This occurs by oxidation of water in the case of oxygenic photosynthesis. The electron-deficient reaction center of photosystem II (P680*) is the strongest biological oxidizing agent known on earth, which allows it to break apart molecules as stable as water.
The water-splitting reaction is catalyzed by the
oxygen evolving complex of photosystem II. This protein-bound inorganic complex contains four manganese ions, plus a calcium and chloride ion as cofactors. Two water molecules are complexed by the manganese cluster, which then undergoes a series of four electron removals (oxidations) to replenish the reaction center of photosystem II. At the end of this cycle, free oxygen (O
2) is generated and the hydrogen of the water molecules has been converted to four protons released into the thylakoid lumen.
These protons, as well as additional protons pumped across the thylakoid membrane coupled with the electron transfer chain, form a
proton gradient across the membrane that drives
photophosphorylation and thus the generation of chemical energy in the form of
adenosine triphosphate (ATP). The electrons reach the
P700 reaction center of
photosystem I where they're energized again by light. They are passed down another electron transfer chain and finally combine with the
coenzyme NADP
+ and protons outside the thylakoids to
NADPH. Thus, the net oxidation reaction of water photolysis can be written as:
2H
2O + 2NADP
+ + 8 photons (light)
2NADPH + 2H
+ + O
2
The free energy change (ΔG) for this reaction is 102 kilocalories per mole. Since the energy of light at 700 nm is about 40 kilocalories per mole of photons, approximately 320 kilocalories of light energy are available for the reaction. Therefore, approximately one-third of the available light energy is captured as NADPH during photolysis and electron transfer. An equal amount of ATP is generated by the resulting proton gradient. Oxygen as a byproduct is of no further use to the reaction and thus released into the atmosphere.
In 2007 a quantum model was proposed by Graham Fleming, which includes the possibility that photosynthetic energy transfer might involve quantum oscillations, explaining its unusually high efficiency.
According to Fleming there's direct evidence that remarkably long-lived wavelike electronic quantum coherence plays an important part in energy transfer processes during photosynthesis, which can explain the extreme efficiency of the energy transfer because it enables the system to sample all the potential energy pathways, with low loss, and choose the most efficient one.
Photolysis in the atmosphere
Photolysis also occurs in the atmosphere as part of a series of reactions by which primary
pollutants such as
hydrocarbons and
nitrogen oxides react to form secondary pollutants such as
peroxyacyl nitrates. See
photochemical smog.
The two most important photodissociaton reactions in the
troposphere are firstly:
» O
3 + hν → O
2 + O(
1D) λ < 320 nm
which generates an excited oxygen atom which can go on to react with water to give the
hydroxyl radical:
» O(
1D) + H
2O → 2OH
The hydroxyl radical is central to
atmospheric chemistry as it initiates the
oxidation of
hydrocarbons in the atmosphere and so acts like a detergent.
Secondly the reaction:
» NO
2 + hν → NO + O
is a key reaction in the formation of
tropospheric ozone.
The formation of the
ozone layer is also caused by photodissociation.
Ozone in the earth's
stratosphere is created by ultraviolet light striking
oxygen molecules containing two oxygen
atoms (O
2), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O
2 to create
ozone, O
3. In addition, photolysis is the process by which
CFCs are broken down in the upper atmosphere to form ozone-destroying chlorine
free radicals.
Astrophysics
In
astrophysics, photodissociation is one of the major processes through which molecules are broken down (but new molecules are being formed). Because of the
vacuum of the
interstellar medium,
molecules and
free radicals can exist for a long time. Photodissociation is the main path by which molecules are broken down. Photodissociation rates are very important in the study of the composition of
interstellar clouds in which
stars are formed.
Typical examples of photodissociation in the interstellar medium are (
is the scientific notation for light, specifically a
photon):
»
»
Multiple photon dissociation
In comparison to ultraviolet or other high energy photons, single photons in the
infrared spectral range usually are not energetic enough for direct photodissociation of molecules. However, after absorption of multiple infrared photons a molecule may gain internal energy to overcome its barrier for dissociation. Multiple photon dissociation (MPD) can be achieved by applying high power lasers, for example a
Carbon dioxide laser, or a
Free electron laser, or by long interaction times of the molecule with the radiation field without the possibility for rapid cooling, for example by collisions. The latter method allows even for MPD induced by
black body radiation.
Further Information
Get more info on 'Photolysis'.
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